Nonvolatile semiconductor storage device

ABSTRACT

A nonvolatile semiconductor storage device includes a plurality of electrode films stacked in a first direction; a silicon pillar piercing the stacked electrode films and separated therefrom by a block insulating film; a charge storage film provided between the block insulating film and the silicon pillar; and a tunnel insulating film provided between the charge storage film and the silicon pillar. The tunnel insulating film comprises a first insulating film having silicon oxide as a base material and containing an added element, wherein a density of the element increases from the silicon pillar toward the charge storage film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 14/606,267 filed Jan. 27, 2015(now U.S. Pat. No. 9,224,875 issued Dec. 29, 2015), which is acontinuation of U.S. Ser. No. 14/157,162 filed Jan. 16, 2014 (now U.S.Pat. No. 8,963,232 issued Feb. 24, 2015), which is a division of U.S.Ser. No. 13/428,111 filed Mar. 23, 2012 (now U.S. Pat. No. 8,674,430issued Mar. 18, 2014), and claims the benefit of priority under 35U.S.C. §119 from Japanese Patent Application No. 2011-195846 filed Sep.8, 2011, the entire contents of each of which are incorporated herein byreference.

FIELD

Embodiments described herein relate generally to a nonvolatilesemiconductor storage device.

BACKGROUND

For example, a structure in which an OTP (onetime-programmable) elementis sandwiched between multilayer interconnections and a structure inwhich a plurality of layers of NAND flash memory is formed by repeatingepitaxial growth of a silicon film have been proposed as technologies torealize higher densities of memory without relying on lithography.However, these structures have a problem of an increasing number oflayers and an increasing number of times of lithography. Thus, athree-dimensional lamination-type vertical memory is proposed.

In a three-dimensional memory, a cylindrical hole (memory hole) isopened collectively in a plurality of electrodes laminated on asemiconductor substrate, a memory film is formed on an inner wall of thehole, and then a polysilicon film (silicon pillar) is formed inside thehole. Accordingly, a memory string serially connected in a laminatingdirection and formed from a plurality of MONOS memory cells can beformed collectively.

In a MONOS memory cell, a challenge is to improve data retaining (chargeretaining) characteristics. Particularly in data retaining afterrepeating write/erase operations, the distribution of threshold voltagein the memory cell spreads, which may make data discriminationdifficult. This applies not only to plane MONOS memory cells, but alsoto cylindrical MONOS memory cells. Particularly in cylindrical MONOSmemory cells, the magnitude of electric field varies in the diameterdirection and a solution of data retaining in consideration of differentelectric fields is needed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an overall configuration example ofa nonvolatile semiconductor storage device according to each embodiment;

FIG. 2 is a perspective view showing a memory cell array in FIG. 1;

FIG. 3 is a sectional view enlarging a NAND string in FIG. 2;

FIG. 4 is a sectional view and a plan view respectively showing a MONOSmemory cell according to a first embodiment;

FIG. 5 is a graph showing the distribution of an N density of the MONOSmemory cell according to the first embodiment;

FIG. 6 is a diagram showing an energy band of the MONOS memory cellaccording to the first embodiment;

FIG. 7 is diagrams showing a comparative example of band modulation bycharge retaining of the MONOS memory cell according to the firstembodiment;

FIG. 8 is diagrams showing the band modulation by charge retaining ofthe MONOS memory cell according to the first embodiment;

FIG. 9 is a graph showing the distribution of an Al density of a MONOSmemory cell according to a second embodiment;

FIG. 10 is a diagram showing the energy band of the MONOS memory cellaccording to the second embodiment;

FIG. 11 is a sectional view and a plan view respectively showing a MONOSmemory cell according to a third embodiment;

FIG. 12 is a diagram showing the energy band of the MONOS memory cellaccording to the third embodiment;

FIG. 13 is a sectional view and a plan view respectively showing a MONOSmemory cell according to a fourth embodiment;

FIG. 14 is a graph showing the distribution of the N density of theMONOS memory cell according to the fourth embodiment;

FIG. 15 is a diagram showing the energy band of the MONOS memory cellaccording to the fourth embodiment;

FIG. 16 is a sectional view showing a MONOS memory cell according to afifth embodiment; and

FIG. 17 is a diagram showing the energy band of the MONOS memory cellaccording to the fifth embodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a nonvolatile semiconductorstorage device comprises a semiconductor substrate; a control gate; ablock insulating film; a charge storage film; a tunnel insulating film,and a semiconductor layer. The control gate is formed on thesemiconductor substrate and includes a cylindrical through holeextending from a top surface to a bottom surface thereof. The blockinsulating film is formed on a side surface of the control gate insidethe through hole. The charge storage film is formed on the side surfaceof the block insulating film inside the through hole. The tunnelinsulating film is formed on a side surface of the charge storage filminside the through hole. The semiconductor layer is formed on a sidesurface of the tunnel insulating film inside the through hole. Thetunnel insulating film comprises a first insulating film having SiO₂ asa base material and containing an element that lowers a band gap of thebase material by being added. A density and a density gradient of theelement monotonously increase from the semiconductor layer toward thecharge storage film.

The present embodiment will be described below with reference todrawings. The same reference numerals are attached to the same elements.A duplicate description may be provided if necessary.

Configuration Example

A configuration example of a nonvolatile semiconductor storage deviceaccording to each embodiment will be described using FIGS. 1, 2, and 3.

FIG. 1 is a perspective view showing an overall configuration example ofa nonvolatile semiconductor storage device according to each embodiment.

As shown in FIG. 1, a nonvolatile semiconductor storage device 100includes a memory cell array 5, a plurality of word line drivingcircuits 13, a plurality of source-side select gate line drivingcircuits 14, a plurality of drain-side select gate driving circuits 15,a sense amplifier 16, a plurality of source line driving circuits 17,and a plurality of back gate transistor driving circuits 18.

In the memory cell array 5, a plurality of word lines WL (control gatesCG), a plurality of bit lines BL, a plurality of source line SL, aplurality of back gates BG, a plurality of source-side select gates SGS,and a plurality of drain-side select gates SGD are provided. In thememory cell array 5, a memory cell transistor MTr that stores data isarranged in each intersection position of the plurality of laminatedword lines WL and a silicon pillar SP in a U shape described later. FIG.1 shows an example in which four layers of the word lines WL arelaminated, but the present embodiment is not limited to this example.

The word line driving circuit 13 is connected to the word line WL tocontrol the voltage applied to the word line WL. Wires connecting theword line driving circuits 13 and the word lines WL are all formed inthe wiring layers at the same level, but the present embodiment is notlimited to this example and may be formed in wiring layers of differentlevels.

The source-side select gate line driving circuit 14 is connected to thesource-side select gate SGS to control the voltage applied to thesource-side select gate SGS.

The drain-side select gate driving circuit 15 is connected to thedrain-side select gate SGD to control the voltage applied to thedrain-side select gate SGD.

The sense amplifier 16 is connected to the bit line BL to amplify apotential read from the memory cell transistor MTr. A bit line drivingcircuit (not shown) controls the voltage applied to the bit line BL.

The source line driving circuit 17 is connected to the source line SL tocontrol the voltage applied to the source line SL. The source linedriving circuit 17 is connected to all the source lines SL, but thepresent embodiment is not limited to this example and one source linedriving circuit 17 may be provided each source line SL.

The back gate driving circuit 18 is connected to the back gate BG tocontrol the voltage applied to the back gate BG.

FIG. 2 is a perspective view showing the memory cell array 5 in FIG. 1and shows the structure of a NAND string (memory cell string) 300. FIG.3 is a sectional view enlarging the NAND string 300 in FIG. 2.

As shown in FIG. 2, a plurality of the NAND strings (memory cellstrings) 300 formed of the U-shaped silicon pillar (semiconductor layer)SP is arranged on a semiconductor substrate 30 in the memory cell array5. Each of the memory strings 300 includes a plurality of the memorycell transistors MTr in which a current path is serially formed alongthe U-shaped silicon pillar SP and two select transistors (a drain-sideselect transistor SDTr and a source-side select transistor SSTr) formedat both ends thereof.

The plurality of the memory cell transistors MTr is formed in eachintersection position of the U-shaped silicon pillar SP and theplurality of the control gates CG and the current path is seriallyconnected along the laminating direction. As shown in FIG. 3, each ofthe memory cell transistors MTr has a memory film 155 between theU-shaped silicon pillar SP and the control gate CG. The memory film 155is formed of a tunnel insulating film 152, a charge storage film 151,and a block insulating film 150 formed around the U-shaped siliconpillar SP in this order. That is, each of the memory cell transistorsMTr is formed of the U-shaped silicon pillar SP and the tunnelinsulating film 152, the charge storage film 151, the block insulatingfilm 150, and the control gate CG formed therearound and has a MONOSstructure. The MONOS structure according to the present embodiment willbe described in detail later.

The drain-side select transistor SDTr is formed in an intersectionposition of the U-shaped silicon pillar SP and the drain-side selectgate SGD. On the other hand, the source-side select transistor SSTr isformed in an intersection position of the U-shaped silicon pillar SP andthe source-side select gate SGS. As shown in FIG. 3, like the memorycell transistor MTr, the drain-side select transistor SDTr and thesource-side select transistor SSTr each have a MONOS structure.

As shown in FIG. 2, the drain-side select transistor SDTr and thesource-side select transistor SSTr are formed above the plurality of thememory cell transistors MTr. One end (drain) of the source-side selecttransistor SSTr is connected to one end (source) of the plurality ofmemory cell transistors and the other end (source) thereof is connectedto the source line SL. On the other hand, one end (source) of thedrain-side select transistor SDTr is connected to the other end (drain)of the plurality of the memory cell transistors MTr and the other end(drain) is connected to the bit line BL.

The U-shaped silicon pillar SP is formed in a U shape in the crosssection in a column direction. The U-shaped silicon pillar SP includes apair of columnar portions extending in the laminating direction and apipe portion formed to be able to connect lower ends of the pair ofcolumnar portions. The pipe portion is provided in the back gate BG toform a back gate transistor BGTr. The U-shaped silicon pillar SP isarranged in such a way that the straight line connecting the center axesof the pair of columnar portions is parallel to the column direction.The U-shaped silicon pillar SP is also arranged so as to be a matrixshape in a plane formed from a row direction and the column direction.Further, as shown in FIG. 3, the U-shaped silicon pillar SP includes ahollow H1 filled with an insulating portion 156.

The plurality of control gates CG is laminated above the back gate BGand is arranged so as to be orthogonal to the columnar portions of theU-shaped silicon pillar SP. Each of the control gates CG extends inparallel with the row direction. Each of the control gates CG is formedso as to be shared by two adjacent columnar portions (two columnarportions on the center side) of four columnar portions in the two memorycell strings 300 adjacent in the column direction. As shown in FIG. 1,edges of the plurality of laminated control gates CG are stepwise and acontact is connected to the top surface of each step. The even-numberedcontrol gates CG in the column direction are mutually connected on oneend in the row direction and the odd-numbered control gates CG aremutually connected on the other end in the row direction.

The back gate BG is formed on the semiconductor substrate 30 (not shown)via an insulating film. The back gate BG is provided below the lowestcontrol gate CG. The back gate BG is formed by extendingtwo-dimensionally in the row direction and the column direction as if tocover a connecting portion of the U-shaped silicon pillar SP.

The drain-side select gate SGD and the source-side select gate SGS areprovided above the highest control gate CG. The drain-side select gateSGD and the source-side select gate SGS extend in the row direction. Thedrain-side select gate SGD is formed so as to be orthogonal to one ofthe columnar portions of the U-shaped silicon pillar SP and thesource-side select gate SGS is formed so as to be orthogonal to theother of the columnar portions thereof. The drain-side select gate SGDand the source-side select gate SGS are formed in a line-and-spacestructure by being insulated and separated each other in the columndirection.

The source line SL is provided above the source-side select gate SGS.The source line SL is formed so as to be shared by two adjacent columnarportions of four columnar portions in the two memory cell strings 300adjacent in the column direction. The source line SL extends in the rowdirection in parallel and is formed as in a line-and-space structure bybeing insulated and separated in the column direction.

The plurality of bit lines BL is provided above the source line SL. Eachof the bit lines BL extends in parallel with the column direction and isformed in a line-and-space structure by being insulated and separatedeach other in the row direction.

First Embodiment

A nonvolatile semiconductor storage device according to the firstembodiment will be described using FIGS. 4, 5, 6, 7 and 8.

In the first embodiment, a tunnel insulating film 152 in a memory celltransistor MTr (MONOS memory cell) is formed of SiO₂ in an interface onthe side of the silicon pillar SP and SiON in the interface on the sideof a charge storage film 151. Then, the tunnel insulating film 152 isconfigured to have an increasing N density therebetween from the side ofthe silicon pillar SP toward the side of the charge storage film 151.Moreover, the gradient of the N density in the tunnel insulating film152 is configured to increase from the side of the silicon pillar SPtoward the side of the charge storage film 151. Accordingly, the bandgap of the tunnel insulating film 152 can be configured to decrease fromthe side of the silicon pillar SP toward the side of the charge storagefilm 151, improving charge retaining characteristics. Also by increasingthe gradient of the N density from the side of the silicon pillar SPtoward the side of the charge storage film 151, the effect of shape inwhich the electric field decreases from the center toward the outercircumference of the cylindrical MONOS memory cell can be compensatedfor to realize a uniform tunnel insulating film electric field.

A nonvolatile semiconductor storage device according to the firstembodiment will be described in detail below.

[Structure]

First, the structure of a MONOS memory cell according to the firstembodiment will be described.

FIGS. 4(a) and 4(b) are a sectional view and a plan view respectivelyshowing a MONOS memory cell according to the first embodiment. Morespecifically, FIG. 4(a) shows a sectional view of a MONOS memory cellaccording to the first embodiment and FIG. 4(b) shows a plan viewthereof.

As shown in FIGS. 4(a) and 4(b), the MONOS memory cell includes acontrol gate CG, a block insulating film 150, the charge storage film151, the tunnel insulating film 152, and the silicon pillar SP.

The control gate CG has a cylindrical memory hole (through hole) 40extending from the top surface to the bottom surface thereof. In otherwords, the control gate CG has the memory hole 40 cutting through in thelaminating direction.

The block insulating film 150 is formed on the side surface of thecontrol gate CG inside the memory hole 40. The block insulating film 150is formed of laminated films of, for example, SiO₂ (silicon oxide), SiN(silicon nitride), and SiO₂ formed from the side surface of the controlgate CG in this order. However, the block insulating film 150 is notlimited to the above example and may be formed of a single film of SiO₂or SiN.

The charge storage film 151 is formed on the side surface of the blockinsulating film 150 inside the memory hole 40. The charge storage film151 is formed of, for example, SiN. However, the charge storage film 151is not limited to the above example and may be formed of variousinsulating films, but is formed of a material whose band gap is smallerthan the band gap of the tunnel insulating film 152 described later.

The tunnel insulating film 152 is formed on the side surface of thecharge storage film 151 inside the memory hole 40. Details of the tunnelinsulating film 152 according to the present embodiment will bedescribed later.

The silicon pillar SP is formed on the side surface of the tunnelinsulating film 152 inside the memory hole 40. The silicon pillar SP isformed of, for example, polysilicon or amorphous silicon. A hollow H1 isformed inside the silicon pillar SP (in the center portion of the memoryhole 40) and is filled with an insulating material. However, the presentembodiment is not limited to the above example and the hollow H1 may beformed as a cavity.

The block insulating film 150, the charge storage film 151, the tunnelinsulating film 152, and the silicon pillar SP are formed along thecylindrical memory hole 40 and are also formed in a cylindrical shape.In addition, the block insulating film 150, the charge storage film 151,the tunnel insulating film 152, and the silicon pillar SP are formedconcentrically around a center O of the memory hole 40.

FIG. 5 is a graph showing the distribution of an N density of the MONOSmemory cell according to the first embodiment. The interface between thechannel (silicon pillar SP) and the tunnel insulating film 152 islocated in a position of a distance of 15 nm from the center O of thememory hole 40. The tunnel insulating film 152 has a thickness of 6 nm.That is, FIG. 5 mainly shows the N density of the tunnel insulating film152. FIG. 6 is a diagram showing an energy band of the MONOS memory cellaccording to the first embodiment.

The tunnel insulating film 152 according to the first embodiment isformed of SiO₂ in the interface with the channel and formed of SiON(silicon oxynitride) in the interface with the charge storage film 151.The tunnel insulating film 152 is also formed of SiON in which the Ndensity changes continuously therebetween. The tunnel insulating film152 has Si (silicon) and O (oxygen), that is, SiO₂ as the base material.The tunnel insulating film 152 also contains N (nitrogen), which isdifferent from the base material.

N is an element that reduces the band gap of the base material by beingadded to the tunnel insulating film 152 having SiO₂ as the base materialthereof. More specifically, in the tunnel insulating film 152, the bandgap thereof can be reduced by increasing the N density thereof and theband gap thereof can be increased by reducing the N density thereof.That is, the band gap of the tunnel insulating film 152 changes as adecreasing function of the N density. The rate of change of the band gapof the tunnel insulating film 152 corresponds to the rate of change ofthe N density.

The base material indicates a main material of the tunnel insulatingfilm 152, but does not simply indicate a material of a large compositionratio. The base material indicates an original material whose band gap(or band offset) is reduced after a different element (here, N) beingadded.

As shown in FIG. 5, the N density in the tunnel insulating film 152monotonously increases from the silicon pillar SP (inside the memoryhole 40) toward the charge storage film 151 (outside the memory hole40). That is, the tunnel insulating film 152 is formed of SiON whose Ndensity increases from the channel toward the charge storage film 151.The gradient (if the density of N is n and the distance from the centerO of the memory hole 40 is r, the differential coefficient thereofdn/dr) of the N density in the tunnel insulating film 152 monotonouslyincreases from the silicon pillar SP toward the charge storage film 151.That is, the N density in the tunnel insulating film 152 has anincreasing rate of change (rate of increase) thereof from the center Oof the memory hole 40 toward the outer side. In this case, the N densityin the tunnel insulating film 152 and the gradient thereof changecontinuously from the silicon pillar SP toward the charge storage film151.

Thus, by introducing an element (N) that is different from the basematerial of the tunnel insulating film 152 whose base material is SiO₂,the band structure of the tunnel insulating film 152 can be modulated.More specifically, as shown in FIG. 6, the tunnel insulating film 152has a large band gap (or band offset) on the channel side and a smallband gap (or band offset) on the side of the charge storage film 151.The band gap (or band offset) in the tunnel insulating film 152monotonously decreases from the silicon pillar SP toward the chargestorage film 151. The absolute value of the gradient of the band gap (orband offset) in the tunnel insulating film 152 monotonously increasesfrom the silicon pillar SP toward the charge storage film 151. In thiscase, the band gap (or band offset) in the tunnel insulating film 152changes continuously from the silicon pillar SP toward the chargestorage film 151. The band offset in the above description is aconduction band offset or a valence band offset.

The composition ratio of the material (SiON) constituting the tunnelinsulating film 152 is represented as (SiO₂)_(x)(Si₃N₄)_(1-x). Morespecifically, x is 1 in the interface with the channel and x is 0.75 inthe interface with the charge storage film 151. That is, the compositionratio changes in the range of x=0.75 to 1. If x is smaller than 0.75 inthe composition ratio, an average coordination number Nav of the tunnelinsulating film 152 exceeds 3. If the average coordination number Navexceeds 3, defects in the film increases rapidly. Thus, x≧0.75 is setfor the composition ratio of a material constituting the tunnelinsulating film 152 in the present embodiment.

Also as shown in FIG. 6, if N is added to the tunnel insulating film 152having SiO₂ as the base material, the modulation on the valence bandside is larger than the modulation on the conduction band side in theenergy band. The modulation on the valence band side can contribute toimprovement of erase characteristics.

[Manufacturing Method]

Next, the manufacturing method of a MONOS memory cell according to thefirst embodiment will be described.

First, the cylindrical memory hole 40 is formed in a conductive layer tobe the control gate CG so as to reach from the top surface to the bottomsurface thereof. Next, the block insulating film 150 is formed on theside surface of the control gate CG inside the memory hole 40. Then, thecharge storage film 151 is formed on the side surface of the blockinsulating film 150 inside the memory hole 40.

Next, the tunnel insulating film 152 is formed on the side surface ofthe charge storage film 151 inside the memory hole 40. The tunnelinsulating film 152 is formed as shown below:

First, a first film formed of SiON is formed on the side surface of thecharge storage film 151 inside the memory hole 40 by, for example, theALD (Atomic Layer Deposition) method. Next, a second film formed of SiONhaving a lower N density than the first film is formed on the sidesurface of the first film by, for example, the ALD method. Then, aplurality of films formed of SiON is similarly formed on the sidesurface of the second film by, for example, the ALD method. In thiscase, an inner film has a lower N density and a smaller gradient than anouter film. Then, lastly, after a film formed of SiO₂ being formed by,for example, the ALD method, annealing is performed sufficiently.Accordingly, N is diffused and the tunnel insulating film 152 in whichthe N density is continuous and the N density and the gradient thereofmonotonously decrease toward the inner side of the memory hole 40 isformed.

Further, the tunnel insulating film 152 is formed as shown below:

First, a film formed of SiN is formed on the side surface of the chargestorage film 151 inside the memory hole 40 by, for example, the ALD andthe film is oxidized to form a first film formed of SiON. Next, a filmformed of SiN is formed on the side surface of the first film by, forexample, the ALD and the film is oxidized to form a second film formedof SiON having the N density lower than the N density of the first film.Then, a film formed of SiN is similarly formed on the side surface ofthe second film by, for example, the ALD and the film is oxidized toform a film formed of SiON and the above process is repeated. In thiscase, an inner film has a lower N density and a smaller gradient than anouter film. Then, lastly, after a film formed of SiO₂ being formed by,for example, the ALD method, annealing is performed sufficiently.Accordingly, N is diffused and the tunnel insulating film 152 in whichthe N density is continuous and the N density and the gradient thereofmonotonously decrease toward the inner side of the memory hole 40 isformed.

Next, the silicon pillar SP is formed on the side surface of the tunnelinsulating film 152 inside the memory hole 40. Then, an insulatingmaterial is formed on the side surface of the silicon pillar SP insidethe memory hole 40 to fill up the center portion thereof to complete aMONOS memory cell.

[Effect]

According to the first embodiment, the tunnel insulating film 152 in aMONOS memory cell is formed of SiO₂ in the interface on the side of thesilicon pillar SP and formed of SiON in the interface on the side of thecharge storage film 151 and the portion therebetween is formed of SiONin which the N density and the gradient of the N density increase(monotonously increase) from the side of the silicon pillar SP towardthe side of the charge storage film 151. Accordingly, the band gap (orthe band offset) of the tunnel insulating film 152 can be configured todecrease from the side of the silicon pillar SP toward the side of thecharge storage film 151. As a result, charge retaining characteristicscan be improved. The principle of the band modulation by chargeretaining will be described below.

FIGS. 7(a) and 7(b) are diagrams showing a comparative example of bandmodulation by charge retaining of the MONOS memory cell according to thefirst embodiment and FIGS. 8(a) and 8(b) are diagrams showing the bandmodulation by charge retaining of the MONOS memory cell according to thefirst embodiment.

According to the comparative example, as shown in FIG. 7(a), if nocharge (electron) is held in the charge storage film 151, there is nointernal electric field (gradient of the band gap) in the tunnelinsulating film 152 and the band gap has a constant magnitude in eachfilm. Then, as shown in FIG. 7(b), if a charge is held in the chargestorage film 151, band modulation arises due to the charge. Morespecifically, a barrier to electrons in the conduction band side becomessmaller due to increased band energy of the charge storage film 151 andalso a self electric field arises in the band energy of the tunnelinsulating film 152 in such a way that electrons are likely to bedetrapped. Accordingly, charge retaining characteristics of the chargestorage film 151 are degraded.

According to the first embodiment, as shown in FIG. 8(a), by contrast,if no charge is held in the charge storage film 151, an internalelectric field arises in the tunnel insulating film 152 and the band gapof the tunnel insulating film 152 decreases from the side of the siliconpillar SP toward the side of the charge storage film 151. Thus, if acharge is held in the charge storage film 151, as shown in FIG. 8(b),even if a self electric field arises in band energy of the tunnelinsulating film 152 due to the charge, the self electric field can bemitigated. That is, detrapping of the charge held in the charge storagefilm 151 can be suppressed.

Also according to the first embodiment, an internal electric fieldarises in the conduction band side so that electrons are acceleratedtoward the side of the charge storage film 151. An electric field duringwriting can thereby be increased, improving write characteristics.Further, an internal electric field arises in the valence band side sothat holes are accelerated toward the side of the charge storage film151. An electric field during erasing can thereby be increased,improving erase characteristics. Particularly, a large internal electricfield is generated in the valence band side by adding N to the tunnelinsulating film 152. Thus, erase characteristics can further beimproved. With improvement in write/erase characteristics, degradationin durability of the memory cell can be suppressed.

Incidentally, an electric field E=C/∈r arises in the diameter directionof the cylinder in a cylindrical MONOS memory cell, where E is theelectric field, ∈ is the dielectric constant, r is the distance from thecenter O of the memory hole 40, and C is a constant proportional to theamount of charge and inversely proportional to the thickness of the wordline WL. That is, the electric field becomes smaller with an increasingdistance from the center O of the memory hole 40 in a cylindrical MONOSmemory cell for write and erase operations. In other words, the electricfield becomes smaller from the center O of the memory hole 40 toward theouter side (shape effect of the cylindrical memory cell).

If the N density is monotonously increased toward the outer side in thediameter direction, the band gap (or the band offset) decreases andthus, an internal electric field can be generated. However, simplyincreasing the N density may not be able to cover a decrease of theelectric field due to the shape effect (increase in distance r) of thecylindrical memory cell and a decrease of the electric field due to anincrease in dielectric constant with the introduction of N. That is, inwrite/erase operations, characteristics thereof cannot be considerablyimproved simply by monotonously increasing the N density in the tunnelinsulating film 152 toward the outer side.

According to the first embodiment, by contrast, not only the N densityin the tunnel insulating film 152 is monotonously increased from theside of the silicon pillar SP toward the side of the charge storage film151, but also the gradient of the N density is monotonously increased.Accordingly, the internal electric field in the side of the chargestorage film 151 can further be increased. As a result, an influence ofthe shape effect of the cylindrical memory cell can be corrected,write/erase characteristics can be improved, and a self electric fielddue to stored charges can sufficiently be mitigated so that satisfactorydata retaining characteristics can be obtained.

Second Embodiment

A nonvolatile semiconductor storage device according to the secondaryembodiment will be described using FIGS. 9 and 10.

In the secondary embodiment, a tunnel insulating film 152 in a memorycell transistor MTr (MONOS memory cell) is formed of SiO₂ in theinterface on the side of a silicon pillar SP and Al₂O₃ in the interfaceon the side of a charge storage film 151. Then, the tunnel insulatingfilm 152 is configured to have an increasing Al density and anincreasing gradient thereof therebetween from the side of the siliconpillar SP toward the side of the charge storage film 151. That is, thesecond embodiment is different from the first embodiment in that,instead of the N density, the Al density is changed.

A nonvolatile semiconductor storage device according to the secondembodiment will be described in detail below. The same aspects in thesecond embodiment as in the first embodiment are omitted and thedescription mainly focuses on differences.

[Structure]

First, the structure of a MONOS memory cell according to the secondembodiment will be described.

FIG. 9 is a graph showing the distribution of an Al (aluminum) densityof a MONOS memory cell according to the second embodiment. The interfacebetween the channel (silicon pillar SP) and the tunnel insulating film152 is located in a position of a distance of 15 nm from a center O of amemory hole 40. The tunnel insulating film 152 has a thickness of 6 nm.That is, FIG. 9 mainly shows the Al density of the tunnel insulatingfilm 152. FIG. 10 is a diagram showing an energy band of the MONOSmemory cell according to the second embodiment.

The tunnel insulating film 152 according to the second embodiment isformed of SiO₂ in the interface on the side of the channel and formed ofAl₂O₃ (alumina) in the interface on the charge storage film 151. Then,the tunnel insulating film 152 is formed of AlSiO (aluminum silicate) inwhich the Al density changes continuously therebetween. The tunnelinsulating film 152 has Si and O, that is, SiO₂ as the base material.The tunnel insulating film 152 also contains the element Al, which isdifferent from the base material.

Al is an element that reduces the band gap of the base material by beingadded to the tunnel insulating film 152 having SiO₂ as the base materialthereof. More specifically, in the tunnel insulating film 152, the bandgap (or the band offset) thereof can be reduced by increasing the Aldensity thereof and the band gap (or the band offset) thereof can beincreased by reducing the Al density thereof. That is, the band gap (orthe band offset) of the tunnel insulating film 152 changes as adecreasing function of the Al density. The rate of change of the bandgap (or the band offset) of the tunnel insulating film 152 correspondsto the rate of change of the Al density.

The base material indicates a main material, but does not simplyindicate a material of a large composition ratio. The base materialindicates an original material whose band gap (or band offset) isreduced after a different element (here, Al) being added.

As shown in FIG. 9, the Al density in the tunnel insulating film 152monotonously increases from the side of the silicon pillar SP toward theside of the charge storage film 151. The gradient (if the density of Alis n and the distance from the center O of the memory hole 40 is r, thedifferential coefficient thereof dn/dr) of the Al density in the tunnelinsulating film 152 monotonously increases from the side of the siliconpillar SP toward the side of the charge storage film 151. That is, theAl density in the tunnel insulating film 152 has an increasing rate ofchange (rate of increase) thereof from the center O of the memory hole40 toward the outer side. In this case, the Al density in the tunnelinsulating film 152 and the gradient thereof change continuously fromthe side of the silicon pillar SP toward the side of the charge storagefilm 151.

Thus, by introducing an element (Al) that is different from the basematerial of the tunnel insulating film 152 whose base material is SiO₂,the band structure of the tunnel insulating film 152 can be modulated.More specifically, as shown in FIG. 10, the tunnel insulating film 152has a large band gap (or band offset) on the channel side and a smallband gap (or band offset) on the side of the charge storage film 151.The band gap (or band offset) in the tunnel insulating film 152monotonously decreases from the side of the silicon pillar SP toward theside of the charge storage film 151. The absolute value of the gradientof the band gap (or band offset) in the tunnel insulating film 152monotonously increases/decreases from the side of the silicon pillar SPtoward the side of the charge storage film 151. In this case, the bandgap (or band offset) in the tunnel insulating film 152 changescontinuously from the side of the silicon pillar SP toward the side ofthe charge storage film 151.

The composition ratio of the material (AlSiO) constituting the tunnelinsulating film 152 is represented as (SiO₂)_(x)(Al₂O₃)_(1-x). Morespecifically, x is 1 in the interface with the channel and x is 0 in theinterface with the charge storage film 151. That is, the compositionratio changes in the range of x=0 to 1.

Also as shown in FIG. 10, if Al is added to the tunnel insulating film152 having SiO₂ as the base material, the modulation on the valence bandside is larger than the modulation on the conduction band side in theenergy band. The modulation on the valence band side can contribute toimprovement of charge retaining characteristics and writecharacteristics.

[Manufacturing Method]

Next, the manufacturing method of a MONOS memory cell according to thesecond embodiment will be described.

First, the cylindrical memory hole 40 is formed in a conductive layer tobe the control gate CG so as to reach from the top surface to the bottomsurface thereof. Next, a block insulating film 150 is formed on the sidesurface of the control gate CG inside the memory hole 40. Then, thecharge storage film 151 is formed on the side surface of the blockinsulating film 150 inside the memory hole 40.

Next, the tunnel insulating film 152 is formed on the side surface ofthe charge storage film 151 inside the memory hole 40. The tunnelinsulating film 152 is formed by the method of forming aluminum silicateusing the ALD method. More specifically, the tunnel insulating film 152is formed as follows:

First, a first film formed of AL₂O₃ is formed on the side surface of thecharge storage film 151 inside the memory hole 40 by, for example, theALD (Atomic Layer Deposition) method. Next, a second film formed of SiO₂is formed on the side surface of the first film by, for example, the ALDmethod. Then, similarly a film formed of AL₂O₃ and a film formed of SiO₂are alternately formed on the side surface of the second film by, forexample, the ALD method.

The ALD method is a deposition method capable of forming an atomic layerin units of layer. Thus, the number of layers of Al₂O₃ inside the memoryhole 40 increases toward to the outer side and decreases toward theinner side. On the other hand, the number of layers of SiO₂ inside thememory hole 40 decreases toward to the outer side and increases towardthe inner side. Then, lastly, after a film formed of SiO₂ being formedby, for example, the ALD method, annealing is performed sufficiently.Accordingly, Al is diffused and the tunnel insulating film 152 in whichthe Al density is continuous and the Al density and the gradient thereofmonotonously decrease toward the inner side of the memory hole 40 isformed.

Next, the silicon pillar SP is formed on the side surface of the tunnelinsulating film 152 inside the memory hole 40. Then, an insulatingmaterial is formed on the side surface of the silicon pillar SP insidethe memory hole 40 to fill up the center portion of the memory hole 40to complete a MONOS memory cell.

[Effect]

According to the second embodiment, the same effect as in the firstembodiment can be achieved. That is, improvement of charge retainingcharacteristics and improvement of write/erase characteristics can besought.

In addition, according to the second embodiment, the tunnel insulatingfilm 152 is formed of SiO₂ in the interface on the side of the siliconpillar SP and formed of Al₂O₃ in the interface on the side of the chargestorage film 151 and a portion therebetween is formed of AlSiO in whichthe Al density and the gradient thereof increase (monotonously increase)from the side of the silicon pillar SP toward the side of the chargestorage film 151. Accordingly, the band gap (or the band offset) of thetunnel insulating film 152 can be configured to decrease from the sideof the silicon pillar SP toward the side of the charge storage film 151.Because a larger internal electric field arises in the conduction bandthan in the valence band, write characteristics can further be improved.

Incidentally, the element added to the tunnel insulating film 152 is notlimited to Al and, for example, Hf (hafnium) may also be added. That is,the tunnel insulating film 152 may be formed of SiO₂ in the interface onthe side of the channel and formed of HfO₂ in the interface on the sideof the charge storage film 151 and a portion therebetween may be formedof HfSiO (hafnium silicate) in which the Hf density increases(monotonously increases) from the silicon pillar SP toward the chargestorage film 151.

Third Embodiment

A nonvolatile semiconductor storage device according to the thirdembodiment will be described using FIGS. 11 and 12.

In the third embodiment, a tunnel insulating film 152 in a memory celltransistor MTr (MONOS memory cell) is formed of a first insulating film152 a containing SiON on the side of a charge storage film 151 and asecond insulating film 152 b containing SiO₂ on the side of a siliconpillar SP. More specifically, the first insulating film 152 a isconfigured to have an increasing N density from the side of the siliconpillar SP toward the side of the charge storage film 151 and the secondinsulating film 152 b is configured by SiO₂ whose composition ratio isconstant. That is, the third embodiment is different from the firstembodiment in that the tunnel insulating film 152 is formed of the firstinsulating film 152 a containing SiON and the second insulating film 152b containing SiO₂.

A nonvolatile semiconductor storage device according to the thirdembodiment will be described in detail below. The same aspects in thethird embodiment as in the first embodiment are omitted and thedescription mainly focuses on differences.

[Structure]

First, the structure of a MONOS memory cell according to the thirdembodiment will be described.

FIGS. 11(a) and 11(b) are a sectional view and a plan view respectivelyshowing a MONOS memory cell according to the third embodiment. Morespecifically, FIG. 11(a) shows a sectional view of a MONOS memory cellaccording to the third embodiment and FIG. 11(b) shows a plan viewthereof.

As shown in FIGS. 11(a) and 11(b), the tunnel insulating film 152 isformed of the first insulating film 152 a and the second insulating film152 b in the third embodiment.

The first insulating film 152 a is formed on the side surface of thecharge storage film 151 inside a memory hole 40. The first insulatingfilm 152 a has a structure similar to the structure of the tunnelinsulating film 152 in the first embodiment. That is, the firstinsulating film 152 a is formed of SiO₂ in the interface with the secondinsulating film 152 b described later and formed of SiON in theinterface with the charge storage film 151. The portion therebetween isformed of SiON in which the N density changes continuously. The firstinsulating film 152 a has Si and O, that is, SiO₂ as the base material.The first insulating film 152 a also contains N, which is different fromthe base material.

The second insulating film 152 b is formed on the side surface of thefirst insulating film 152 a inside the memory hole 40. The secondinsulating film 152 b is formed of SiO₂. The composition ratio of thesecond insulating film 152 b is constant. The second insulating film 152b has a thickness of 1 nm or more. The silicon pillar SP is formed onthe side surface of the second insulating film 152 b. In other words,the second insulating film 152 b is formed between the first insulatingfilm 152 a and the silicon pillar SP.

FIG. 12 is a diagram showing the energy band of the MONOS memory cellaccording to the third embodiment.

As shown in FIG. 12, the band gap in the first insulating film 152 amonotonously decreases from the side of the silicon pillar SP toward theside of the charge storage film 151. The absolute value of the gradientof the band gap in the first insulating film 152 a monotonouslyincreases from the side of the silicon pillar SP toward the side of thecharge storage film 151. In this case, the band gap in the firstinsulating film 152 a changes continuously from the side of the siliconpillar SP toward the side of the charge storage film 151.

The composition ratio of the material (SiON) constituting the firstinsulating film 152 a is represented as (SiO₂)_(x)(Si₃N₄)_(1-x). Morespecifically, x is 1 in the interface with the second insulating film152 b and x is 0.75 in the interface with the charge storage film 151.That is, the composition ratio changes in the range of x=0.75 to 1.

On the other hand, the second insulating film 152 b has a constantmagnitude. This is because the second insulating film 152 b does notcontain N, is formed of SiO₂, and is formed in a constant compositionratio. The band gap in the second insulating film 152 b has a magnitudecomparable to the magnitude of the band gap of edges on the side of thesilicon pillar SP (inside the memory hole 40) of the first insulatingfilm 152 a. Thus, the band gap of the first insulating film 152 a andthe band gap of the second insulating film 152 b are continuously incontact. In other words, the first insulating film 152 a and the secondinsulating film 152 b are in contact with the continuous compositionratio.

[Manufacturing Method]

Next, the manufacturing method of a MONOS memory cell according to thethird embodiment will be described.

First, the cylindrical memory hole 40 is formed in a conductive layer tobe the control gate CG so as to reach from the top surface to the bottomsurface thereof. Next, a block insulating film 150 is formed on the sidesurface of the control gate CG inside the memory hole 40. Then, thecharge storage film 151 is formed on the side surface of the blockinsulating film 150 inside the memory hole 40.

Next, the first insulating film 152 a is formed on the side surface ofthe charge storage film 151 inside the memory hole 40. The firstinsulating film 152 a is formed by a method almost the same as themethod of forming the tunnel insulating film 152 in the firstembodiment.

Then, the second insulating film 152 b is formed on the side surface ofthe first insulating film 152 a inside the memory hole 40. The secondinsulating film 152 b formed of SiO₂ is formed by forming the firstinsulating film 152 a formed of SiON and, for example, annealing edgesthereof (inside the memory hole 40) with oxygen.

The second insulating film 152 b is also formed by forming the firstinsulating film 152 a formed of SiON, then forming polycrystallinesilicon and, subsequently annealing the back side of the polycrystallinesilicon (inside the memory hole 40). That is, the front side of thepolycrystalline silicon is oxidized by supplying oxygen viapolycrystalline silicon to be the silicon pillar SP, forming the secondinsulating film 152 b between the first insulating film 152 a and thesilicon pillar SP.

Then, the center portion of the memory hole 40 is filled up by a normalmethod to complete a MONOS memory cell.

[Effect]

According to the third embodiment, the same effect as in the firstembodiment can be achieved. That is, improvement of charge retainingcharacteristics and improvement of write/erase characteristics can besought.

According to the third embodiment, the tunnel insulating film 152 isformed of the first insulating film 152 a containing SiON on the side ofthe charge storage film 151 and the second insulating film 152 bcontaining SiO₂ on the side of the silicon pillar SP. That is, there isa region (the second insulating film 152 b) in which N is not presentbetween the channel and the first insulating film 152 a. If N is presentnear the channel, mobility of charges in the channel is degraded.According to the third embodiment, mobility of charges in the channelcan be enhanced by forming the second insulating film 152 b having no Nin the interface with the channel.

The first insulating film 152 a is not limited to a structure similar tothe structure of the tunnel insulating film 152 in the first embodimentand may have a structure similar to the structure of the tunnelinsulating film 152 in the second embodiment. That is, the firstinsulating film 152 a may be formed of SiO₂ in the interface on the sideof the second insulating film 152 b and formed of Al₂O₃ in the interfaceon the side of the charge storage film 151 and the portion therebetweenmay be formed of AlSiO in which the Al density and the gradient thereofincrease (monotonously increase) from the side of the second insulatingfilm 152 b toward the side of the charge storage film 151.

Fourth Embodiment

A nonvolatile semiconductor storage device according to the fourthembodiment will be described using FIGS. 13, 14 and 15.

The fourth embodiment is a modification of the third embodiment and anexample in which a tunnel insulating film 152 in a memory celltransistor MTr (MONOS memory cell) is formed of a third insulating film152 d containing SiON, a fourth insulating film 152 e containing SiONhaving a lower N density than the N density of the third insulating film152 d, and a fifth insulating film 152 f containing SiO₂ laminated inthe above order from the side of a charge storage film 151. The fourthembodiment is different from the third embodiment in that the thirdinsulating film 152 d, the fourth insulating film 152 e, and the fifthinsulating film 152 f constituting the tunnel insulating film 152 arefilms having discontinuous composition ratios, instead of films havingcontinuous composition ratios.

A nonvolatile semiconductor storage device according to the fourthembodiment will be described in detail below. The same aspects in thefourth embodiment as in the third embodiment are omitted and thedescription mainly focuses on differences.

[Structure]

First, the structure of a MONOS memory cell according to the fourthembodiment will be described.

FIGS. 13(a) and 13(b) are a sectional view and a plan view respectivelyshowing a MONOS memory cell according to the fourth embodiment. Morespecifically, FIG. 13(a) shows a sectional view of a MONOS memory cellaccording to the fourth embodiment and FIG. 13(b) shows a plan viewthereof.

As shown in FIGS. 13(a) and 13(b), the tunnel insulating film 152 isformed of the third insulating film 152 d, the fourth insulating film152 e, and the fifth insulating film 152 f.

The third insulating film 152 d is formed on the side surface of thecharge storage film 151 inside a memory hole 40. The third insulatingfilm 152 d is formed of SiON. The third insulating film 152 d has Si andO, that is, SiO₂ as a base material and also contains N, which isdifferent from the base material. The N density in the third insulatingfilm 152 d is, for example, 18 at. %. The third insulating film 152 dhas a constant composition ratio and has a thickness of, for example, 2nm.

The fourth insulating film 152 e is formed on the side surface of thethird insulating film 152 d inside the memory hole 40. The thirdinsulating film 152 d is formed of SiON. The fourth insulating film 152e has Si and O, that is, SiO₂ as a base material and also contains N,which is different from the base material. The N density in the fourthinsulating film 152 e is lower than the N density in the thirdinsulating film 152 d and is, for example, 7 at. %. The fourthinsulating film 152 e has a constant composition ratio and has athickness of, for example, 2 nm.

The fifth insulating film 152 f is formed on the side surface of thefourth insulating film 152 e inside the memory hole 40. The fifthinsulating film 152 f is formed of SiO₂. The fifth insulating film 152 fhas a constant composition ratio. The fifth insulating film 152 f has athickness of, for example, 2 nm. A silicon pillar SP is formed on theside surface of the fifth insulating film 152 f. The fifth insulatingfilm 152 f has Si and O, that is, SiO₂ as a base material and alsocontains N, which is different from the base material.

Incidentally, the relationship of N density in the third insulating film152 d, the fourth insulating film 152 e, and the fifth insulating film152 f is not limited to the above example and only needs to be asfollows.

FIG. 14 is a graph showing the distribution of the N density of theMONOS memory cell according to the fourth embodiment.

As shown in FIG. 14, a case when the N densities of the third insulatingfilm 152 d, the fourth insulating film 152 e, and the fifth insulatingfilm 152 f are n₁, n₂, and n₃ and the thicknesses of the thirdinsulating film 152 d, the fourth insulating film 152 e, and the fifthinsulating film 152 f are r₁, r₂, and r₃ respectively will beconsidered.

In this case, the N densities satisfy the relationship n₁>n₂>n₃.Gradients of the N densities satisfy the relationship(n₁−n₂)/(r₁+r₂)>(n₂−n₃)/(r₂+r₃). That is, the N density in the tunnelinsulating film 152 increases with an increasing distance from thecenter O of the memory hole 40. The difference (gradient) of the Ndensities between adjacent films also increases with an increasingdistance from the center O of the memory hole 40. More specifically, thedifference in N density between the fourth insulating film 152 e and thethird insulating film 152 d positioned on the outer side is larger thanthe difference in N density between the fifth insulating film 152 f andthe fourth insulating film 152 e positioned on the inner side.

Incidentally, if r₁=r₂=r₃ (thicknesses are all equal) and n₃=0, theequation n₁>2×n₂ holds.

If there are four or more insulating films, instead of three, conditionsof the N densities and N density gradients hold. More specifically, acase when the N densities of insulating films formed in the order fromthe outer side (charge storage film 151 side) toward the inner side(silicon pillar SP side) are n₁, n₂, . . . , n_(i) (i is an integer andi≧3) and the thicknesses thereof are r₁, r₂, . . . , r_(i) will beconsidered. In this case, the N densities satisfy the relationship n₁>n₂. . . >n_(i). Gradients of the N densities satisfy the relationship(n₁−n₂)/(r₁+r₂)>(n₂−n₃)/(r₂+r₃) . . . >(n_(i−1)−n_(i))/(r_(i−1)+r_(i)).

FIG. 15 is a diagram showing the energy band of the MONOS memory cellaccording to the fourth embodiment.

As shown in FIG. 15, the band gap of the third insulating film 152 d hasa constant magnitude. This is because the third insulating film 152 d isformed of SiON having a constant composition ratio.

The band gap of the fourth insulating film 152 e has a constantmagnitude and is larger than the band gap of the third insulating film152 d. This is because the fourth insulating film 152 e is formed ofSiON having a constant composition ratio and the N density thereof islower than the N density of the third insulating film 152 d.

The band gap of the fifth insulating film 152 f has a constantmagnitude. This is because the fifth insulating film 152 f does notcontain N, is formed of SiO₂, and is formed with a constant compositionratio. The fifth insulating film 152 f has a larger band gap than thethird insulating film 152 d and the fourth insulating film 152 e becausethe fifth insulating film 152 f does not contain N.

As shown in FIG. 15, the band gap increases in the order of the thirdinsulating film 152 d, the fourth insulating film 152 e, and the fifthinsulating film 152 f. In other words, the tunnel insulating film isformed of laminated films with an increasing band gap from the chargestorage film 151 toward the silicon pillar SP. Moreover, the band gapsof the third insulating film 152 d, the fourth insulating film 152 e,and the fifth insulating film 152 f are stepwise in each interface. Thatis, as shown in FIG. 14, composition ratios of the third insulating film152 d, the fourth insulating film 152 e, and the fifth insulating film152 f are stepwise and discontinuous in each interface.

[Manufacturing Method]

Next, the manufacturing method of a MONOS memory cell according to thefourth embodiment will be described.

First, the cylindrical memory hole 40 is formed in a conductive layer tobe the control gate CG so as to reach from the top surface to the bottomsurface thereof. Next, a block insulating film 150 is formed on the sidesurface of the control gate CG inside the memory hole 40. Then, thecharge storage film 151 is formed on the side surface of the blockinsulating film 150 inside the memory hole 40.

Next, the third insulating film 152 d formed of SiON is formed on theside surface of the charge storage film 151 inside the memory hole 40by, for example, the ALD method. Next, the fourth insulating film 152 ehaving a lower N density than the third insulating film 152 d and formedof SiON is formed on the side surface of the third insulating film 152 dinside the memory hole 40 by, for example, the ALD method. Next, thefifth insulating film 152 f formed of SiO₂ is formed on the side surfaceof the fourth insulating film 152 e inside the memory hole 40 by, forexample, the ALD method. Accordingly, the tunnel insulating film 152formed of laminated films with discontinuous composition ratios isformed.

Next, the silicon pillar SP is formed on the side surface of the tunnelinsulating film 152 inside the memory hole 40. Then, an insulatingmaterial is formed on the side surface of the silicon pillar SP insidethe memory hole 40 to fill up the center portion of the memory hole 40to complete a MONOS memory cell.

[Effect]

According to the fourth embodiment, the same effect as in the thirdembodiment can be achieved. That is, improvement of charge retainingcharacteristics and improvement of write/erase characteristics can besought and also mobility of charges in the channel can be enhanced.

According to the fourth embodiment, the tunnel insulating film 152 isformed of the third insulating film 152 d containing SiON, the fourthinsulating film 152 e containing SiON having a lower N density than theN density of the third insulating film 152 d, and the fifth insulatingfilm 152 f containing SiO₂ laminated in the above order from the side ofthe charge storage film 151. The third insulating film 152 d, the fourthinsulating film 152 e, and the fifth insulating film 152 f are eachfilms having a constant composition ratio. In addition, the compositionratio in each interface is discontinuous. That is, according to themanufacturing method in the fourth embodiment, the tunnel insulatingfilm 152 is formed by performing the ALD method using gas types of threepatterns. A switching sequence of the ALD method occurs less frequently.Moreover, there is no need of subsequent annealing. Accordingly,processes can be made simpler and the processing time can be reduced.

In the fourth embodiment, a case when the tunnel insulating film 152 hasthree layers is described, but the tunnel insulating film 152 may havefour layers or more. The thickness of each of the third insulating film152 d, the fourth insulating film 152 e, and the fifth insulating film152 f is set to 2 nm, but may be changed when appropriate if the totalthickness of the tunnel insulating film 152 is about 6 nm.

Fifth Embodiment

A nonvolatile semiconductor storage device according to the fifthembodiment will be described using FIGS. 16 and 17.

The fifth embodiment is a modification of the third embodiment and anexample in which a tunnel insulating film 152 in a memory celltransistor MTr (MONOS memory cell) is formed of a sixth insulating film152 g containing SiON on the side of a charge storage film 151 and aseventh insulating film 152 h containing SiO₂ on the side of a siliconpillar SP and an Si microcrystal 152 i is contained near an interfacethereof.

A nonvolatile semiconductor storage device according to the fifthembodiment will be described in detail below. The same aspects in thefifth embodiment as in the third embodiment are omitted and thedescription mainly focuses on differences.

[Structure]

First, the structure of a MONOS memory cell according to the fifthembodiment will be described.

FIG. 16 is a sectional view showing a MONOS memory cell according to thefifth embodiment.

As shown in FIG. 16, a tunnel insulating film 152 includes the sixthinsulating film 152 g, the seventh insulating film 152 h, and the Simicrocrystal 152 i in the fifth embodiment.

The sixth insulating film 152 g is formed on the side surface of thecharge storage film 151 inside a memory hole 40. The sixth insulatingfilm 152 g has a structure similar to the structure of the firstinsulating film 152 a in the third embodiment.

The seventh insulating film 152 h is formed on the side surface of thesixth insulating film 152 g inside a memory hole 40. The seventhinsulating film 152 h has a structure similar to the structure of thesecond insulating film 152 b in the third embodiment.

The Si microcrystal 152 i is contained near the interface between thesixth insulating film 152 g and the seventh insulating film 152 h in thetunnel insulating film 152. The existing density of the Si microcrystal152 i is desirably constant, but may be different from region to region.In FIG. 16, the Si microcrystal 152 i is formed only near the interfacebetween the sixth insulating film 152 g and the seventh insulating film152 h, but may be present inside the seventh insulating film 152 h. TheSi microcrystal 152 i is also called an Si dot or an Si nano-crystal.

FIG. 17 is a diagram showing the energy band of the MONOS memory cellaccording to the fifth embodiment.

As shown in FIG. 17, the band gap in the sixth insulating film 152 gmonotonously decreases from the side of the silicon pillar SP toward theside of the charge storage film 151. The absolute value of the gradientof the band gap in the sixth insulating film 152 g monotonouslyincreases from the side of the silicon pillar SP toward the side of thecharge storage film 151. In this case, the band gap in the sixthinsulating film 152 g changes continuously from the side of the siliconpillar SP toward the side of the charge storage film 151.

On the other hand, the band gap in the seventh insulating film 152 h hasa constant magnitude. This is because the second insulating film 152 bdoes not contain N, is formed of SiO₂, and is formed with a constantcomposition ratio.

If N is added to the sixth insulating film 152 g having SiO₂ as a basematerial, the modulation on the valence band side is larger than themodulation on the conduction band side in the energy band.

In the fifth embodiment, the Si microcrystal 152 i is added to the sixthinsulating film 152 g having SiO₂ as a base material. In this case, asshown in FIG. 17, an influence of the introduction of the Simicrocrystal 152 i becomes evident in the energy band and theprobability of tunneling of electrons in the conduction band side duringwriting increases. Further, the probability of tunneling of holes in thevalence band side during erasing increases.

That is, not only improvement of erase characteristics, but also furtherimprovement of charge retaining characteristics and writecharacteristics can be sought.

[Manufacturing Method]

Next, the manufacturing method of a MONOS memory cell according to thefifth embodiment will be described.

First, the cylindrical memory hole 40 is formed in a conductive layer tobe the control gate CG so as to reach from the top surface to the bottomsurface thereof. Next, a block insulating film 150 is formed on the sidesurface of the control gate CG inside the memory hole 40. Then, thecharge storage film 151 is formed on the side surface of the blockinsulating film 150 inside the memory hole 40.

Next, the sixth insulating film 152 g is formed on the side surface ofthe charge storage film 151 inside the memory hole 40. The sixthinsulating film 152 g is formed by a method similar to the method offorming the first insulating film 152 a in the third embodiment.

Next, a silicon thin film is formed on the side surface of the sixthinsulating film 152 g inside the memory hole 40. Then, RTA (RapidThermal Anneal) processing is performed, for example, at 900 to 1000°.The RTA processing is performed at high temperature for a short time toform the Si microcrystal 152 i. In this manner, the Si microcrystal 152i is formed on the surface of the sixth insulating film 152 g.

Then, the seventh insulating film 152 h is formed on the side surface ofthe sixth insulating film 152 g on which the Si microcrystal 152 i isformed. The seventh insulating film 152 h is formed by a method similarto the method of forming the second insulating film 152 b in the thirdembodiment.

Then, the center portion of the memory hole 40 is filled up by a normalmethod to complete a MONOS memory cell.

[Effect]

According to the fifth embodiment, the same effect as in the thirdembodiment can be achieved. That is, improvement of charge retainingcharacteristics and improvement of write/erase characteristics can besought and also mobility of charges in the channel can be enhancedbecause the insulating film near the channel is formed of SiO₂.

According to the fifth embodiment, the tunnel insulating film 152 isformed of the sixth insulating film 152 g containing SiON on the side ofthe charge storage film 151 and the seventh insulating film 152 hcontaining SiO₂ on the side of the silicon pillar SP and the Simicrocrystal 152 i is contained near an interface thereof. By adding theSi microcrystal 152 i to the tunnel insulating film 152, the probabilityof electron tunneling particularly in the conduction band sideincreases, improving write characteristics. That is, not only N thatcauses a large internal electric field in the valence band side, butalso the Si microcrystal 152 i that improves write characteristics isadded to the tunnel insulating film 152. Thus, not only improvement oferase characteristics by adding N, but also further improvement ofcharge retaining characteristics and write characteristics can besought.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A nonvolatile semiconductor storage device,comprising: a stacked structural unit including a plurality of electrodefilms stacked in a first direction; a silicon pillar piercing thestacked structural unit; a block insulating film provided between theelectrode films and the silicon pillar; a charge storage film providedbetween the block insulating film and the silicon pillar; and a tunnelinsulating film provided between the charge storage film and the siliconpillar, wherein the tunnel insulating film includes silicon oxide as abase material and includes an element, the tunnel insulating filmincludes a first point, a second point, and a third point in this orderfrom the silicon pillar toward the charge storage film, and a firstdensity of the element at the first point is lower than a second densityof the element at the second point and the second density of the elementis lower than a third density of the element at the third point.
 2. Thedevice of claim 1, wherein the first point is located at an interfacebetween the tunnel insulating film and the silicon pillar and the thirdpoint is located at an interface between the tunnel insulating film andthe charge storage film.
 3. The device of claim 2, wherein the secondpoint is located at middle with the first point and the third point. 4.The device of claim 1, wherein the element of the tunnel insulating filmis nitrogen.
 5. The device of claim 1, wherein the element of the tunnelinsulating film is one of aluminum and hafnium.
 6. The device of claim1, further comprising: a first insulating film provided between thetunnel insulating film and the silicon pillar, the first insulating filmbeing containing silicon oxide.
 7. The device of claim 1, wherein adensity gradient between the first point and the second point is smallerthan a density gradient between the second point and the third point. 8.The device of claim 1, wherein the charge storage film is a siliconnitride film.
 9. The device of claim 1, wherein the tunnel insulatingfilm has a composition ratio of (SiO₂)_(x)(Si₃N₄)_(1-x), where(0.75≦x≦1).
 10. The device of claim 1, wherein a density of the elementmonotonously increases from the silicon pillar toward the charge storagefilm.
 11. The device of claim 1, further comprising: a first insulatinglayer being provided on an inner side of the silicon pillar.
 12. Anonvolatile semiconductor storage device, comprising: a stackedstructural unit including a plurality of electrode films stacked in afirst direction; a silicon pillar piercing the stacked structural unit;a block insulating film provided between the electrode films and thesilicon pillar; a charge storage film provided between the blockinsulating film and the silicon pillar; and a tunnel insulating filmprovided between the charge storage film and the silicon pillar, whereinthe tunnel insulating film includes silicon oxide as a base material andincludes nitrogen, the tunnel insulating film includes a first point, asecond point, and a third point in this order from the silicon pillartoward the charge storage film, and a first density of nitrogen at thefirst point is lower than a second density of nitrogen at the secondpoint and the second density of nitrogen is lower than a third densityof nitrogen at the third point.
 13. The device of claim 12, wherein thefirst point is located at an interface between the tunnel insulatingfilm and the silicon pillar and the third point is located at aninterface between the tunnel insulating film and the charge storagefilm.
 14. The device of claim 13, wherein the second point is located atmiddle with the first point and the third point.
 15. The device of claim12, further comprising: a first insulating film provided between thetunnel insulating film and the silicon pillar, the first insulating filmbeing containing silicon oxide.
 16. The device of claim 12, wherein adensity gradient between the first point and the second point is smallerthan a density gradient between the second point and the third point.17. The device of claim 12, wherein the charge storage film is a siliconnitride film.
 18. The device of claim 12, wherein the tunnel insulatingfilm has a composition ratio of (SiO₂)_(x)(Si₃N₄)_(1-x), where(0.75≦x≦1).
 19. The device of claim 12, wherein a density of nitrogenmonotonously increases from the silicon pillar toward the charge storagefilm.
 20. The device of claim 12, further comprising: a first insulatinglayer being provided on an inner side of the silicon pillar.
 21. Thedevice of claim 1, wherein the element lowers a band gap of the basematerial by being added.